I seem to remember a story where he's considered the "black wizard" and he's the only one who can get to ME. He saves Boromir, and destroys the Uruk-Hai at Helm's Deep. He keeps going back and forth between ME and Hogwarts, or something like that.

It's been a while, but I don't remember any Dragonball Z in it.

I think you--or I--may be confusing two badfics--"Sanctuary of Arda" is the LotR/HP/Dragonball Z crossover, like Eni said. There's another fic, "The Black Wizard of Middle-earth" or something like that where he goes to Beleriand, becomes Harold the Black, and makes Fëanor OOC. I could be wrong, though.

"Sanctuary of Arda" ? It seems abandoned, but the story is here: http://www.fictionalley.org/authors/keiran_halcyon/SOA.html

I'd be no help with your second question, though. It's been a while since I read LotR.